The most obvious distinction Barbara A. Mikulski will take into retirement is that she’s spent more time in Congress than any other woman, and that’s a record worthy of significant recognition. But, especially at a Capitol so deeply mired in dysfunction and partisanship, the meaning of her service is deeper than mere longevity.

Mikulski has become the embodiment of "old school" in an institution where the thrill of the new has taken hold with a vengeance. Beyond rattling so many glass ceilings during her four decades on the Hill, Maryland’s senior Democratic senator has stuck with all manner of virtues and behaviors that have fallen into disfavor by the newer members — devoted as they are to confrontation and content to claim deadlock as their principal work product. She operates on the belief that seniority should not only guarantee additional power, but should also create an obligation to advise, teach and promote those in the younger generation of lawmakers savvy enough to seek help from their elders. That's why one of her lasting contributions will be her mentoring of the female senators, from both parties, who arrived after her.

Some of them refer to her as “the dean,” others have dubbed her “Coach Barb.” And for more than a decade, Mikulski’s main venue for cultivating the careers and preserving the sanity of those in the sisterhood of senators has been convening a monthly dinner that’s strictly off the record, where the conversation over wine and home-state products ranges from work-life balance to legislative brainstorming. Attendees say the unusual bipartisan “zone of civility” Mikulski created may be one of her most important legacies — because it’s incubated the sort of genuinely collegial, non-ideological professional friendships that are nearly extinct in the current Congress. These are the sort of bonds that, at least into the 1990s, were essential to making legislative compromise the norm rather than the exception.

A product of rowhouse life and clubhouse politics, Mikulski is unambiguous in her view that government should be an engine of social justice, tuned particularly to the needs of blue-collar communities such as Baltimore's Fells Point, where she grew up. But her operating premise is different from many on the Democratic left, or the Republican right: Good legislating means testing your political comfort zone in search of partners on deals that benefit all sides. “I’m a believer in coalitions,” is one of her mantras.

She worked assiduously to promote such bargaining during the divided 113th Congress, when she held the gavel of Senate Appropriations. But she was largely stymied in pursuing her simple-sounding goal : To give Congress, the federal bureaucracy and the voting public a break from several years of budgetary chaos with “a return to regular order” in the annual production of spending bills.

She does not suffer fools gladly — really at all — and is known for being a particularly demanding boss, even in the Capitol’s universe of 535 demanding bosses. (Washingtonian magazine’s annual survey of congressional aides has pegged her “meanest senator” several times.) But she has proved to be fiercely supportive of staffers and others who live up to her high standards, and she says her reputation for intimidation is no different from the kind of tough-love that’s historically been a virtue in the male-dominated Senate.

Mikulski also leavened her brusque cred with some of the funnier and more self-deprecating turns of phrase in recent memory. Just in the past year, for example, she’s described her timetable for moving spending bills as “bodacious,” labeled the most politically problematic of those measures her “ugly stepsisters” and pleaded with colleagues to advance one stuck bill she dubbed “the little engine that could.”

When her departure, which she announced Monday , becomes official at the end of next year, Mikulski will have been a member for precisely four decades. She had only recently celebrated her 40th birthday when she was elected to represent the heart of Baltimore in the House in 1976, and she’ll turn 80 next summer, a few months before her fifth and final term as a Maryland senator concludes.

Three years ago she broke the record for longest-serving female member that had been set by Edith Nourse Rogers, a House Republican from Massachusetts for 35 years until her death in 1960. And Mikulski has also notched several additional milestones. She was the first female Democrat to serve in both chambers; the first woman in the Senate Democratic leadership, having held the No. 3 position of caucus secretary for a decade ending in 2004; and the first woman to be chairwoman of a congressional Appropriations panel. She’s also the only current member in the National Women’s Hall of Fame, having been inducted in 2011.

She was only the 16th female senator ever when she arrived in 1987 — and only the seventh chosen by election instead of appointment. But it took until 2007 before there were as many females in the Senate as there had been in all of American history when Mikulski took office. (This year there are 20, all but four of them Democrats.)

Along the way, and while working to preserve some of the Senate’s best practices, Mikulski also swept away some of the place’s most hidebound customs. In the early 1990s, for example, she and the other female senator at the time, Republican Nancy Kassebaum of Kansas, orchestrated a quiet protest against the policy that all women wear dresses or skirts on the Senate floor. They arranged to wear trousers one day and urged all female staffers to join them — and pants have been a gender-neutral part of the senatorial uniform ever since.

That anecdote is maybe a tiny part of what Vice President Joseph R. Biden Jr. had in mind when he reacted to Monday’s news this way: “There’s a lot of talk about what the women of America owe Barbara Mikulski, but the truth of the matter is the men of America owe her even more. Because she freed men of the stereotypical notions that they were raised to believe."